Mechanical Characteristics
• 25-pin connector
– 9-pin connector is more commonly found in IBM-PC but it covers signals for asynchronous serial communication only
• Use male connector on DTE and female connector on DCE • Note: all signal names are viewed from DTE
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Function of Signals
• DCD: data carrier detect
– ON when two modems have negotiated successfully and the carrier signal is established on the phone line
• RTS: request to send
– ON when DTE wants to send data – Used to turn on and off modem’s carrier signal in multi-point (i.e. multi-drop) lines – Normally constantly ON in point-to-point lines
• CTS: clear to send
– ON when DCE is ready to receive data
• SG: signal ground
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g. in order to benefit from modem’s data compression protocol) » without flow control.6kbps.2kbps and line speed = 33. terminal speed = 115.Flow Control
• Means to ask the transmitter to stop/resume sending in data • Required when:
– DTE to DCE speed > DCE to DCE speed (e. the buffer within modem will overflow – sooner or later – the receiving end takes time to process the data and thus cannot be always ready to receive
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because terminal speed > link speed
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. it activates CTS – normally used between computer and modem » computer is always ready to receive data but modem is not.Hardware Flow Control
• RTS/CTS
– the transmitting end activates RTS to inform the receiving end that it has data to send – if the receiving end is ready to receive.

Xoff is sent to the transmitting end to ask it to stop – when data have been processed by the receiving end and the buffer has space again. a graphics file) contains a byte equal to 13H (Xoff)
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. Xon is sent to the transmitting end to notify it to resume – advantage: only three wires are required (TD. RD and GND) – disadvantage: confusion arises when the transmitted data (e.Software Flow Control
• Xon/Xoff
– when the buffer within the receiving end is nearly full.g.